It is known from the prior art to use one or more extended-nip presses in a press live section that follows the forming section of a paper machine. In such an extended-nip press, the length of the pressing zone extending in the running direction of the paper web is substantially longer than that encountered in a roll press nip. Traditionally, extended-nip presses were commonly used only in paperboard machines and to make thicker grades of paper. However, more recently, extended-nip presses have also been introduced for use in the production of thinner paper grades, such as newspaper and fine papers.
Known press sections have various problems, especially when used in the high speed manufacture of relatively thin paper products. Relatively thin paper products are much more likely to be damaged during their manufacture than are thicker paper products, especially at relatively high web speeds of about 25 to about 35 m/sec. The relatively low number of intertwined fibers due to the very thinness of such thin paper products is one cause of such vulnerability. This latter factor also contributes to the difficulties encountered when attempting to separate such relatively thin paper stock from relatively smooth surfaces because considerable adhesion forces have to be overcome. Consequently, the incidence of tearing of and other damage to relatively thin paper webs is much higher even in such currently available press sections that employ extended nip presses than that encountered with thicker materials.
Examples of prior art that is considered to be most closely related to the present invention can be found in the following published patent documents: Finnish patent application No. 890530 (corresponding to U.S. Pat. No. 4,923,570 and published German patent document No. DE-OS 3808293.4), German utility model No. DE-GBM 8805966, Finnish patent application No. 913886 (corresponding to U.S. Pat. No. 5,178,732 and published German patent document No. DE-OS 4026021), Finnish Pat. No. 75,382, Finnish patent application No. 811403 (corresponding to U.S. Pat. No. 4,440,598), and U.S. Pat. Nos. 4,257,844, 4,551,205, 4,704,192, and 5,120,399. Further, reference is made to the paper in the journal Wochenblatt fur Papierfabnkation 19(1993), pages 780 to 782"Die Flexonip.RTM.-Pressen", which describes some of the latest extended nip press constructions of J. M. Voith GmbH. The press sections in these prior art devices have various problems.
In Finnish patent application No. 890,530, especially with reference to FIGS. 3 and 4 (and in the corresponding U.S. Pat. No. 4,923,570), the paper web is transferred from one extended nip press into a second and/or third extended nip press on the smooth surface of a gliding belt mantle of the extended nip press. Because the gliding belt mantle is smooth, water is pressed out of the paper web only through the surface of the paper web which faces away from the mantle because the smooth surface of the mantle has no route by which water can leave the paper web. Additionally, such a smooth surface mantle has pronounced adhesion properties and therefore poor paper web transfer properties. Moreover, the first nip is not an extended nip that removes water efficiently through both surfaces of the paper web, and the overall concept disclosed is not that of a compact press section of several nips.
In the FIGS. 1 and 2 embodiments in German utility model No. DE-GMB 88 05 966, the press section consists solely of two separate extended-nip presses in which the high load pressing is carried out on the same face of the same pressing felt. Moreover, the paper web is transferred over the long distance between the two nips by confining the paper web between two wet felts and on the face of one felt, thus resulting in the rewetting of the web by water previously expelled from the paper web to the felt. Consequently, the water content of the paper web increases after it is pressed in this so-called rewetting process. While the geometries of the press sections that are shown in FIGS. 3, 5 and 6 are more compact than those of FIGS. 1 and 2, the same press felt runs through both of the extended nips, thereby reducing the water absorbing capacity of the felt in the second extended nip. The transfer-of the paper web from one extended nip to the other relies fully on the differences in the surface characteristics of the pressing felts as there is no additional mechanism to ensure the transfer of the paper web.
In Finnish patent application No. 913886 (corresponding to U.S. Pat. No. 5,178,732), FIGS. 1, 1a, 2, 3, 3a and 3b, a transfer means, which lower than that of a felt, runs through all the press nips. Since the largest amount of water is removed in the first nip, the dewatering in one direction (i.e. through one surface of the paper web) is limited by the transfer means contacting that surface. In FIGS. 2, 2a, 3, 3a and 3b, the transfer means in the first nip consists of a gliding belt which is impervious to water and smooth so that dewatering through one surface of the paper web is completely precluded.
Finnish patent application No. 811403 (corresponding to U.S. Pat. No. 4,440,598), Finnish Pat. No. 75,382, and U.S. Pat. No. 4,257,84 do not disclose or suggest the use of an extended nip press with a high dewatering capacity at the first nip or anywhere else, or a closed web transfer to the dryer section.
U.S. Pat. No. 5,120,399 merely discloses the use of a single felt extended nip at the first nip. In this case, water is removed from the paper web in one direction only. Also, the press sections comprise only two nips, and no compact multi-nip press section is suggested.